01 - Preheat oven to 375°F. Position the oven rack in the lower third of the oven. Line a baking sheet with parchment paper.
02 - In a large mixing bowl, combine the rhubarb, strawberries, blueberries, and raspberries. Add granulated sugar, brown sugar, cornstarch, cinnamon, vanilla extract, and lemon juice. Toss gently until all the fruit is evenly coated. Set aside.
03 - Gently press the unbaked pie crust into a 9-inch pie dish. Trim any overhanging dough and crimp the edges to your preference.
04 - Spoon the berry-rhubarb mixture evenly into the prepared pie crust, distributing the fruit in an even layer.
05 - In a medium bowl, whisk together the flour, rolled oats, brown sugar, salt, and cinnamon. Add the cold butter cubes and use your fingers or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
06 - Sprinkle the crumble topping evenly over the fruit filling, covering the surface completely.
07 - Place the pie on the prepared baking sheet and bake for 50 to 55 minutes, until the topping is golden brown and the filling is bubbling around the edges. If the crumble begins browning too quickly, tent loosely with aluminum foil after the first 30 minutes.
08 - Allow the pie to cool for at least 2 hours before slicing so the filling has time to set. Serve warm or at room temperature, optionally with vanilla ice cream or freshly whipped cream.